As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a single mode glass fiber having a core doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source. The pump energy which is within the absorption band of the rare earth dopant is combined with the optical input signal within the coupler and applied to the medium, and an amplified output signal is emitted from the other end of the fiber.
Such amplifiers are typically used in a variety of applications including, but not limited to, amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed.
One type of fiber amplifier referred to as an erbium doped fiber amplifier (EDFA) typically includes a silica fiber having a single-mode core doped with erbium (specifically doped with erbium ions conventionally denoted as Er.sup.3+). It is well known that an EDFA operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1550 nm. Since 1550 nm is the lowest loss wavelength of conventional single-mode glass fibers, erbium amplifiers are well suited for inclusion in fiber systems that propagate optical signals having wavelengths around 1550 nm.
For many communications applications, particularly those involving the use of digital signal transmission, it is necessary to use polarized optical signals. Modulators used for digital signal modulation use changing signal polarization to control the generation of digital pulse, and are therefore polarization dependent. Thus, it is necessary to have polarized optical energy to ensure that the desired signal modulation is accomplished. Using optical signal energy with a known, controlled polarization is therefore highly desirable for optical signal communication applications.
It has been an ongoing pursuit in the field of optical amplifiers to increase the power output of the amplifiers and/or reduce the noise figure (i.e., the additional noise introduced into the input signal by the amplifier itself). It is therefore an object of this invention to provide an optical amplifier having better performance in output power and relative noise than those demonstrated in the prior art.